Why is the sky blue?

Rayleigh Scattering. http://en.wikipedia.org/wiki/Rayleigh_scattering

Basically the dust particles in the air cause the some of the shortest wavelengths of light (ie Blue, indigo and violet) to scatter in all directions. The rest of the light is not scattered by the dust. At sunset you can see the other colors that are not scattered. If you look closely you might be able to see all the colors of the rainbow. (Red Orange, Yellow, Green, Blue, Indigo and Violet)

"The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue."
http://www.sciencemadesimple.com/sky_blue.html

Because grass is green. If the sky as green too, them we wouldn't be able to tell the difference an we would live in, what we would see as a big box. Not to mentional all the problems that airplanes would have to deal with, pilots all disoriented...

@fuse1921: What about planes flying over the ocean?

Thats where the term 'uncharted waters' would come in.

I saw a program about Mercury. The sky is black there because the planet has very little gravity and as a result very little atmosphere. When light passes through our atmosphere it appears blue.

The theory of slickrick can be applied to the sea aswell:
"Why is the water close to the beach green-ish, and far away dark blue?"

There is still sand under the sea, which is yellow, and the water is transculent, but also a little bit blue. And I thought if you merge blue and yellow, you get green. And how deeper the water gets, how less yellow (you can't see the bottom, it's dark and the water blocks vision) is added to the blue of the water, so it's blue.

I realized this at school, when I had handcraft.
(I don't know the real good word, but I'm sure you get it :-))

Example: http://rurl.org/7ya

Davey.
Please, correct me if I'm wrong...